State of the art systems for exhaust gas recirculation in motor vehicles are known. With the aid of such systems, nitrogen oxides entrapped in the exhaust gases, in particular in the exhaust gases of diesel-powered motor vehicles, can be reduced and fuel consumption of gasoline-powered motor vehicles can be lowered. In generic systems of exhaust gas recirculation, either cooled or uncooled exhaust gas is added to the fresh air being drawn in by the engine.
During combustion at high temperatures, in particular when lean mixtures are employed in the partial-load operational range, environmentally harmful nitrogen oxides are created in the engines of motor vehicles. In order to reduce nitrogen oxide emissions, it is necessary to decrease the high temperature peaks and to reduce the amount of excess air during combustion. By means of the lower oxygen concentration of the fuel-air mixture, the speed of the combustion process and thus the maximum combustion temperatures are reduced. Both effects are attained by the mixture of a partial flow of the exhaust gas to the flow of fresh air which is drawn in by the engine.
In diesel-powered motor vehicles, apart from the reduction of the oxygen content and the temperature peaks during combustion, a system of exhaust gas recirculation also leads to a reduction of noise emissions. In gasoline-powered motor vehicles comprising an exhaust gas recirculation system, throttling losses are minimized.
However, the admixture of the recirculated exhaust gas flow at high temperatures leads to a reduction of the cooling effect and thus of the efficiency of the engine. In order to counteract said reductions, the exhaust gas is cooled in a so-called exhaust gas heat exchanger or exhaust gas recirculation cooler prior to admixture. In gasoline-powered motor vehicles, the additional cooling of the exhaust gas leads to an increase of the compression ratio of the air being supplied to the engine.
Embodiments of exhaust gas heat exchangers are known. However, increasingly stringent legislation with respect to emission standards and consumption requirements for motor vehicles presuppose an increased cooling need in face of an ever-decreasing space requirement for the components in the vehicle. These conflicting requirements are only rarely fulfilled by known exhaust gas heat exchangers.
German Pat. No. DE 10 2007 054 953 A1 discloses an exhaust gas recirculation system of an internal combustion engine having an air-cooled exhaust gas recirculation cooler. The exhaust gas recirculation cooler, which is made of aluminum, features two-pass cooling tubes which lead into single-pass connection ports. By means of distributing the exhaust gas flow over two cooling tubes, the heat transfer surface is enlarged, thus resulting in an enhanced cooling capacity. The two-pass cooling tubes, which are additionally connected to one another via cooling fins, are wound three times in a U-shape.
German Pat. No. DE 10 2007 054 913 A1 describes a heat exchanger, in particular for a motor vehicle, having one or several flow ducts through which a fluid can flow. The flow ducts, which are provided in an extrusion profile, furthermore, at least in some sections feature a curved profile, in order to increase the heat transfer efficiency. According to one embodiment of the heat exchanger, the extrusion profiles are designed so as to be bent in a U-shape. A coolant flows around the outer walls of the extrusion profiles, while the exhaust gas flows along the inner wall.
In German Pat. No. DE 10 2008 024 569 A1 an exhaust gas cooler having a housing with a bypass duct and a cooling zone is disclosed. In the cooling zone, an exhaust gas cooling duct is disposed, which is formed by straight cooling tubes and deflection chambers. The housing comprises a control member for controlling the exhaust gas flow either by means of the bypass duct or else by means of the cooling zone. The exhaust gas flow is deflected during passage through the cooling zone, wherein the exhaust gas cooling duct features an inlet cooling duct, an adjoining deflection duct and an outlet cooling duct in turn adjoining the deflection duct. The exhaust gas flow thereby flows in the deflection duct counter to the flow direction of the inlet or outlet cooling duct. The exhaust gas flow to be cooled is directed at least four times through the cooling zone of the housing. The coolant flows around the cooling tubes, while the exhaust gas flows through the cooling tubes.
The exhaust gas recirculation systems known in the art comprise gas/gas and gas/water heat exchangers, wherein the gas/water heat exchangers are formed in particular as tube bundle heat exchangers, which in turn are embodied as pure I-flow or U-flow exhaust gas heat exchangers. The pure I-flow heat exchangers with the arrangement of the gas inlet and the gas outlet along one line, exhibit low pressure losses at the exhaust gas side, however, along with a low cooling capacity. In the U-flow exhaust gas heat exchangers, the gas inlet and the gas outlet are arranged on one side of the heat exchanger. As a result of the exhaust gas flowing out of the tubes into deflection chambers and subsequently into the tubes, however, high pressure losses occur on the exhaust gas side when a good cooling capacity is to be realized.